The thickness evolution of orthorhombic lattice distortions in heteroepitaxial La_0.67Ca_0.33MnO₃/NdGaO₃(110)_Orobserved by x-ray reciprocal space mapping

Abstract

La0.67Ca0.33MnO3 (LCMO) films of 6–60 nm thickness were grown epitaxially on orthorhombic NdGaO3(1 1 0)Or (NGO) substrates by the pulsed laser deposition method. Like NGO, the films when relaxed should also have an orthorhombic structure that can be described by a pseudocubic perovskite unit, with the angle β between a and c axes deviating from 90°. Using high-resolution off-specular x-ray reciprocal space mapping, we clearly observed the angle deviations in pseudocubic LCMO(0 0 1)/NGO(0 0 1) and investigated the thickness evolution of lattice distortions in the films. At above 30 nm the films are angularly relaxed, while the thinner ones suffer from the shear strain. We argue that for this system although the lattice mismatch in between is negligible the shear strain is crucial for transport properties of the ultrathin LCMO films.

Contrasting size-scaling behavior of ferromagnetism in La0.67Ca0.33MnO3 films and La0.67Ca0.33MnO3/CaRuO3 multilayers

“…Specifically, the films at 1.6 and 2.4 nm are insulating even at 9 T, the film at 3.2 nm becomes metallic at 105 K after applying H of 3 T, and the films at 4.8, 9.6, and 32 nm show the MIT at T P ¼ 180, 209, and 238 K, respectively, consistent with the thickness effect reported previously. [13][14][15][16][17][18][19][20][21] Correspondingly, the MR {¼[q(0 T) À q(9 T)]/q(0 T)} is shown in the bottom panels, and it is seen that for x > 3.2 nm the T MR is roughly equal to the T P .…”

“…1, were fabricated on NdGaO 3 (110) [NGO(110)] substrates by pulsed laser deposition, with each layer grown under the same parameters as previously described. 12 Since all the layers and the substrate are orthorhombic with the same Pbnm symmetry and negligible average lattice mismatch, high-quality epitaxy can be achieved for each film as revealed by high-resolution xray diffractions (XRDs) including linear scans and the offspecular reciprocal space maps (not shown), [12][13][14][15][16] using CuK a1 radiation (k ¼ 1.5406 Å , Panalytical X'pert). The resistivity (q) was measured on a Quantum Design physical property measurement system, and the magnetic property on a vibrating sample magnetometer, with the magnetic field (H) applied along the in-plane [À110] direction.…”

“…The discrepancy between the different groups was ascribed to the various annealing conditions. 13,14 The LCMO/SrTiO 3 (001) [STO(001)] films suffer a larger epitaxial strain and thus, a more suppressed T C . 13 For our as-grown LCMO films, albeit with a gap of $25 K in T C (cf.…”

“…It is well documented that there exists an interfacial "dead layer" that is AF-insulating, although the origin remains controversial. [13][14][15][16][17][18][19][20] The epitaxial strain imposed by the substrates can distort the interfacial octahedra (size, shape, and connectivity), which would affect the exchange interactions by altering the Mn-O-Mn bond length and angle. 1,21 It can also favor a preferential occupation of the Mn e g orbital, inducing the phase instability in manganites.…”

“…16 But, this effect failed to explain the steady decrease of T C for the commensurate films at nanometer scale. 14,15 For LCMO/NGO(110) films, the negligible lattice mismatch (0.04% along [001] and 0.05% along [À110]) precludes the dominant role of the lattice strain, instead the suppression of FM may be attributed to the electronic phase separation (EPS) occurring at both the interface and surface of LCMO. The H-dependent q-T behavior of the 3.2 nm film shown in Fig.…”